The present invention relates generally to magnetic resonance imaging and more particularly relates to magnetic resonance imaging using spatial modulation of magnetization to establish a frame of reference in the form of tag lines on the resulting image.
Magnetic resonance imaging (MRI is a well established tool for medical imaging. In magnetic resonance imaging (MRI), the proton density of physical tissue characteristics is sensed to discriminate between healthy and diseased tissue. In the case of tissue that is in motion, such as the human heart, diseased tissue of the heart can be detected by analysis of wall motion of a heart chamber and wall thickness during the cardiac cycle.
U.S. Pat. No. 4,953,554 describes a system and method for MRI imaging which uses a modulated RF signal to impose reference void lines on the resulting image. The ""554 patent recognizes that such non-invasive MRI tagging is useful in establishing a frame of reference in the image data which is important in the analysis of moving bodies, such as the heart. The method described in the ""554 patent includes modulating an RF pulse that is applied to the body being imaged prior to acquiring image data. More particularly, the ""554 patent discloses modulating an RF carrier pulse in the form (1+2 cos xcfx890t) with a function in the form of (sin(t)/t) just prior to conducting imaging acquisition. However, this form of modulated signal results in a non-uniform gradient which can result in aliasing.
There have been a number of improvements developed to spatially modulate the magnetization field, thereby providing a reference frame on the image in a non-invasive manner. Most notable among these improvements are the SPAMM and DANTE tagging techniques.
The SPAMM method, which is described for example in the article xe2x80x9cA cine-SPAMM Sequence for NMR Imaging of Pulsatile Cerbrospinal Fluid Flow,xe2x80x9d by Wayte et. al., Phys. Med. Biol., pp. 455-463, 1993, produces a sinusoidal modulation of saturated magnetization. An improvement to SPAMM, know as high-order SPAMM, improves the resulting modulation profile by employing binomial modulation of the RF pulse train. While these systems have proven effective in a number of applications, neither SPAMM nor high order SPAMM provide the user with the flexibility to alter the ratio of the tag width (width of the void line) to tag separation. Thus, the user is limited in the form of the resulting image tagging that can be achieved.
DANTE tagging employs a delta function to effect tagging. By effecting spatial modulation with a series of narrow RF pulses of uniform amplitude, the resulting saturation tags exhibit a SINC profile. The DANTE system is further described, for example, in the article entitled xe2x80x9cMyocardial Tagging with B1 insensitive Adiabatic DANTE Inversion Sequencesxe2x80x9d by Tesekos, et al., 1994, pp 395-401, which is hereby incorporated by reference in its entirety.
It is an object of the present invention to provide MRI tagging using a modulated RF pulse train which yields a rectangular profile of saturated magnetization.
It is a further object of the present invention to provide a method of MRI tagging which provides flexibility in selecting the resulting tag width and tag separation.
In accordance with the present invention, a first method for inducing tag lines in an MRI image begins with the steps of determining a desired tag line width and determining a desired tag line separation. One or more parameters of a SINC modulating function are then selected in accordance with the desired tag line width. One or more parameters of an RF pulse train based are selected based upon the desired tag line separation. A gradient field is applied to the specimen being imaged. The RF pulse train is modulated in accordance with the SINC modulating function and the modulated RF pulse train is applied to the specimen being imaged prior to an image acquisition operation.
Preferably, the SINC modulating function is defined at least in part by the bandwidth of a main lobe, and by altering the bandwidth of the main lobe the width of the resulting tag lines is altered accordingly. It is also preferable for the RF pulses of the RF pulse train to be separated by a delay time between pulses which can be altered to determine the resulting tag line separation. Because these parameters of the modulating waveform and RF pulse train can be varied substantially independently of each other, a great deal of flexibility is provided.
Each pulse in the RF pulse train has a flip angle associated with a pulse amplitude and pulse duration. Preferably, the pulse amplitude and pulse duration can be varied, in an inverse relationship, to maintain the flip angle associated with the pulse while optimizing or reducing the peak power requirement. In one embodiment, the pulses in the RF pulse train have a common amplitude and the duty cycle of the pulses is modulated to deliver the appropriate flip angle per pulse in accordance with the SINC modulating function.
In the methods set forth above, the step of applying the gradient preferably occurs in a pulsed manner such that the gradient is active only between pulses of the RF pulse train.